Understanding and preventing HLA-associated drug reactions Immunologically-mediated adverse drug reactions (IM-ADRs) contribute disproportionately to drug- related morbidity and mortality and the cost and uncertainty of drug development. T-cell mediated drug hypersensitivity reactions are a subset of IM-ADEs that are severe and life-threatening, causing severe skin disease such as Stevens-Johnson Syndrome/toxic epidermal necrolysis (SJS/TEN), and organ failure. Severe T-cell mediated drug hypersensitivity syndromes have recently been associated with specific class I and/or II HLA alleles which has led to translational pathways for screening and prevention as well as great insight into their immunopathogenesis. The best example is the current widespread use of the HLA class I allele HLA- B*57:01 as a screening test prior to abacavir prescription in routine HIV clinical practice. We have made significant progress in defining the mechanistic basis of the predisposition of HLA-B*57:01 carriers to abacavir hypersensitivity which now has created a translational roadmap to define further the immunopathogenetic mechanisms of other severe HLA-associated T-cell mediated drug hypersensitivity syndromes. Notably, this recent work based on the abacavir model suggests that drugs rapidly and non-covalently bind to an HLA allele and alter the repertoire of self-peptides binding to the allele, creating a vigorous CD8+ T cell response. Abacavir specific CD8+ T-cell responses can be reproduced in-vitro in 100% of HLA-B*57:01 positive abacavir- nave healthy donors. A key question central to the mechanism of HLA-associated IM-ADRs, and not explained by the altered peptide repertoire model, is why in vivo hypersensitivity generally occurs in only a small proportion of those carrying an HLA-risk allele and what determines the organ specificity of the hypersensitivity. This understanding is integral to the development of prediction and prevention models for severe T-cell mediated drug hypersensitivity. We will address this fundamental question through parallel studies of the T-cell receptor (TCR) usage and the T-cell antigen specificities of the relevant T cells. In Specific Aim 1 we will identify the primary TCR used by T cells in precisely phenotyped drug hypersensitive patients but not HLA-matched drug tolerant controls. We will then define in Specific Aim 2 anti-viral T cells directed against chronic prevalent human herpes viruses (HHV) present in these drug hypersensitive patients but not HLA-risk allele matched drug-tolerants and will focus on stored cell samples from HLA-B*57:01 positive abacavir hypersensitive and HLA-B*15:02 positive SJS/TEN patients. Finally in Specific Aim 3 to test the heterologous immune model, we will identify anti-viral T cells in drug hypersensitive patients that cross- recognize drug in the context of the defined HLA risk allele. The results of these studies will inform strategies for the prediction of severe HLA-associated IM-ADRs and guide drug development and design.